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Method for identification of grouting coninuity of rock bolts |
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Method for identification of grouting coninuity of rock bolts Author: A. Staniek
A rock bolt which is grouted underground may not be properly inserted which results in discontinuity of a resin layer surrounding it. Such discontinuity may also occur in working conditions due to typical rock behavior and displacement. It may be hazardous. In this paper a method for non-destructive identification of discontinuity of a resin layer surrounding rock bolts is presented. The method uses modal analysis procedures and is based on an impact excitation where a response transducer is positioned at a visible part of a rock bolt. As the installed rock bolt acts as an oscillator, different lengths of discontinuity of resin layer change its modal parameters. By proper extraction of these parameters, from which a resonant frequency is seen as the most valuable, the intended identification is possible. At the first phase of research work measurements and analyses were performed in laboratory conditions on models with different types of discontinuity of a resin layer. A special stand was prepared and rock bolts were grouted into resin cylinders of different lengths and clinched to a 20-t foundation. Though there was a good correlation between results obtained quite large interaction with supporting elements was observed. Not being unexpected it proved that such laboratory conditions can not be used as a reference for in situ measurements. What also was gained from laboratory measurements and analysis was the fact that most significant role in identification of different lengths of discontinuity of a resin layer, which form boundary conditions, play natural frequencies. Damping does not convey satisfactory information on that subject and may vary to a certain degree from sample to sample over shadowing its proper usefulness. Since tests in real conditions are performed on relatively short length of a rock bolt, a mode shape usage was also constrained. One of the obstacles of in situ measurements, especially in severe environmental conditions, is the lack of a relatively low cost apparatus to be used for acquiring and recording data. These severe conditions are frequently met in coal mines where rock bolts are installed to support roof sections. Because of this inventing and constructing of a portable measurement system was invented and constructed. As a programming tool the LabVIEW program was used and as hardware a DAQ card installed on a laptop platform and worked out conditioning signal unit were inserted to the measurement chain. As a continuation of laboratory measurements and analysis relevant tests were performed at an experimental coal mine underground. One of technical problems was proper insertion of the rock bolt into a roof section with known length and location of its discontinuity. After overcoming this obstacle underground tests were realized and through relevant analysis modal parameters, especially natural frequencies, were derived. The experiment setup is shown in Figure 3. Proper identification of the natural frequencies of the grouted rock bolt is a key point in determining a position and length of the discontinuity of a resin layer. But what is also needed is a reference point, with which our results may be compared. With this aim theoretical modal analysis was introduced and a base of Finite Element (FE) models were built for different types of discontinuities (different boundary conditions). Then the process of reconciliation of these two groups of models was undertaken and comparison between relevant known cases of discontinuities was made. An example of a Finite Element model and an analytical mode shape for such boundary conditions are presented in Figure 9. As there are many problems connected with proper reconciliation, exact experimental estimation of the physical properties of structural materials, including rock properties, was done. At the final stage many tests on unknown cases of discontinuity of a grout layer were performed. One of the example was the case when a particular rock bolt was exposed to explosion of dynamite charge localized in adjacent area in a coal mine roadway. The test was performed before and after the explosion. Satisfactory results were obtained. To sum up, the method consists mainly of two parts: proper derivation of modal parameters of an examined rock bolt, with natural frequencies seen as most valuable, and comparison with the data base of Finite Element models. The observed conclusions are: close relation between modal parameters and boundary conditions of tested structures exists, satisfactory repeatability for the same cases of discontinuity was obtained.
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2011